1. Field of the Invention
The present invention relates to an image sensor and an image capturing apparatus having the image sensor and, more specifically, to an image sensor capable of capturing a still image and/or a moving image using a number of photoelectric conversion units that are arranged two-dimensionally, and an image capturing apparatus for performing phase difference focus detection using the image sensor.
2. Description of the Related Art
For an electronic camera capable of recording a moving image or a still image, there has been proposed a technique of implementing focus detection by a phase difference detection method using an image sensor for image recording. In the phase difference detection method, a light beam that has passed through the exit pupil of a photographing optical system is split into two light beams. The two split light beams are received by a set of light-receiving element groups for focus detection. The shift amount between the signal waveforms of a pair of images which are two images output in accordance with the light receiving amounts, that is, the relative positional shift amount generated in the pupil division direction of the light beam is detected, thereby obtaining the focus shift amount (defocus amount) of the photographing optical system. The focus detection characteristic of this method depends on the array of the focus detection pixels or the pupil division characteristic shapes of the pixels. Hence, various techniques have been proposed concerning the intra-pixel structure or array to improve the focus detection characteristic.
On the other hand, the image sensor is anticipated to acquire a high-resolution image containing little noise. For this purpose, each pixel of the image sensor preferably receives a light beam having passed through a region as wide as possible in the exit pupil of the photographing optical system. However, using the light beam in the wide pupil region may conflict with improving the performance in phase difference focus detection. To satisfy both the image capturing ability and the phase difference detection ability, the following techniques have been proposed.
In, for example, Japanese Patent Laid-Open No. 2007-158692, each pixel of the image sensor has a first photoelectric conversion unit arranged in the central region of the pixel and a second photoelectric conversion unit arranged around it. An image signal is generated using the output of the first photoelectric conversion unit, and phase difference focus detection is performed using the output of the second photoelectric conversion unit.
In Japanese Patent Laid-Open No. 2009-015164, a plurality of pixel groups having different split center positions of photoelectric conversion units are provided to ensure redundancy for a change in the exit pupil position of the photographing optical system. An optimum pixel group is selected in accordance with the exit pupil position, thereby reducing unbalance of the light receiving amount of the focus detection signal.
In Japanese Patent Laid-Open No. 2007-279312, two types of focus detection pixels are provided independently of imaging pixels. A distance w3 between the gravity centers of distance measurement pupils in the pupil arrangement direction of one type of focus detection pixels is made different from a distance w3 between the gravity centers of distance measurement pupils in the pupil arrangement direction of the other type of focus detection pixels. There is disclosed selecting one of the two types of focus detection pixels based on the magnitude of the defocus amount.
However, in the technique disclosed in Japanese Patent Laid-Open No. 2007-158692, since the pixel arrangement emphasizes the image capturing characteristic, a satisfactory focus detection characteristic is not necessarily obtained. For example, for a photographing optical system having a large f-number, that is, a small exit pupil diameter, the light beam to the photoelectric conversion unit for focus detection is vignetted, and focus detection may be impossible. In addition, in the peripheral portion of the image sensor, that is, in the region with a large image height, the exit pupil diameter becomes small due to vignetting of the photographing optical system. The vignetting state changes depending on the model of the photographing optical system, the zoom state and focus state. Hence, a focus-detectable region also changes depending on these states, making stable focus detection difficult.
In the technique disclosed in Japanese Patent Laid-Open No. 2009-015164, since pupil division is limited in one direction, focus detection cannot be performed for an object having a luminance distribution only in a direction orthogonal to it. To increase the device sensitivity for image acquisition, the area of the photoelectric conversion unit needs to be large. However, in a large defocus state, the blur of the focus detection image also becomes large, and the focus-detectable defocus range narrows.
In Japanese Patent Laid-Open No. 2007-279312, the two types of focus detection pixels do not serve as imaging pixels and therefore become defective pixels when acquiring an image.
On the other hand, when an image sensor having a pupil division ability is used, a 3D image having parallax information can be acquired. However, how to optimize both the focus detection ability and the 3D image acquisition ability is unknown even when the techniques described in Japanese Patent Laid-Open Nos. 2007-158692 and 2009-015164 are used.